Wireless communication systems are well known in the art. Generally, such systems comprise communication stations, which transmit and receive wireless communication signals between each other. Depending upon the type of system, communication stations typically are one of two types of wireless transmit/receive units (WTRUs): one type is the base station, the other is the subscriber unit, which may be mobile.
The term base station as used herein includes, but is not limited to, a base station, access point, Node B, site controller, or other interfacing device or WTRU in a wireless environment, that provides other WTRUs with wireless access to a network with which the AP is associated.
The term wireless transmit/receive units (WTRU) as used herein includes, but is not limited to, a user equipment, mobile station, fixed or mobile subscriber unit, pager, or any other type of device capable of operating in a wireless environment. Such WTRUs include personal communication devices, such as phones, video phones, and Internet ready phones that have network connections. In addition, WTRUs include portable personal computing devices, such as PDAs and notebook computers with wireless modems that have similar network capabilities. WTRUs that are portable or can otherwise change location are referred to as mobile units.
Typically, a network of base stations is provided wherein each base station is capable of conducting concurrent wireless communications with appropriately configured WTRUs, as well as multiple appropriately configured base stations. Some WTRUs may alternatively be configured to conduct wireless communications directly between each other, i.e., without being relayed through a network via a base station. This is commonly called peer-to-peer wireless communications. Where a WTRU is configured to communicate directly with other WTRUs it may itself also be configured as and function as a base station. WTRUs can be configured for use in multiple networks, with both network and peer-to-peer communications capabilities.
One type of wireless system, called a wireless local area network (WLAN), can be configured to conduct wireless communications with WTRUs equipped with WLAN modems that are also able to conduct peer-to-peer communications with similarly equipped WTRUs. Currently, WLAN modems are being integrated into many traditional communicating and computing devices by manufacturers. For example, cellular phones, personal digital assistants, and laptop computers are being built with one or more WLAN modems.
Popular WLAN environments with one or more WLAN base stations, typically called access points (APs), include those constructed according to one or more of the IEEE 802 family of standards. Access to these networks usually requires user authentication procedures. Protocols for such systems are presently being standardized in the WLAN technology area. One such framework of protocols is represented by the IEEE 802 family of standards.
A basic service set (BSS) is the basic building block of an IEEE 802.11 WLAN, which comprises WTRUs also referred to as stations (STAs). A set of STAs which can talk to each other can form a BSS. Multiple BSSs are interconnected through an architectural component called a distribution system (DS), to form an extended service set (ESS). An access point (AP) is a WTRU that provides access to the DS by providing DS services, and generally allows concurrent access to the DS by multiple STAs.
A network of WTRUs operating with peer to peer communications in an IEEE 802.11 environment, typically referred to as “ad hoc” mode, is also called an “independent BSS.” In an independent BSS, two or more WTRUs establish communication among themselves without the need of a coordinating network element. No AP-to-network infrastructure is required. However, an AP can be configured to use the ad hoc protocols and act as the WTRUs do in peer to peer communications. In such case an AP may act as a bridge or router to another network or to the Internet.
A WTRU that starts an ad hoc network selects the ad hoc network's operating parameters, such as the service set identifier (SSID), channel, and beacon timing, and transmits this information in communication frames, for example, in beacon frames. As other WTRUs join the ad hoc network, they detect and use the ad hoc network's operating parameters.
Where a network infrastructure is used and wireless communications are controlled through APs, parameters such as the SSID are normally specified by a network controller associated with the APs. The APs periodically broadcast beacon frames to enable WTRUs to identify the APs and attempt to establish communications with them.
The SSID in an IEEE 802 based system can be a 32-character unique identifier attached to a header of packets sent over a WLAN. The SSID then acts as a password when a WTRU attempts to connect to a BSS or an independent BSS. The SSID differentiates one WLAN from another, so all base stations and all other devices connected to or attempting to connect to a specific WLAN normally use the same SSID. A device will not normally be permitted to join a BSS unless it can provide the correct SSID.
In an AP-based WLAN, a mobile WTRU communicates wirelessly with a specific AP when in the geographic service area of that AP. The WTRU is said to be associated with the AP through which it is conducting wireless communications. It is sometimes necessary or desirable for a WTRU to change the AP with which it is associated. For example, the WTRU may be experiencing poor signal conditions because it is moving out of the geographic area served by the AP with which it is associated. The need to associate to another AP can also be caused by congestion arising in the basic service set (BSS) served by that AP. Re-association is also referred to as roaming, particularly if the WTRU has an assigned “home” AP (or network of APs) and then “roams” when the WTRU communicates via a different AP (or different network's AP).
In WLANs compliant with the current 802.11 standards, the re-association of a WTRU to a new AP is initiated entirely by the WTRU. To accomplish this, a WTRU must first identify APs in its vicinity that can potentially provide network services to the WTRU. This is conventionally achieved by a scanning process, which may be active or passive.
In active scanning, the WTRU transmits probe requests on one or more channels on which the WTRU is configured to communicate. The WTRU selects a new AP from among the APs from which it receives probe responses. In passive scanning, the WTRU dwells on the channels it can use in an attempt to receive beacon packets transmitted by APs which serve the geographic location where the WTRU is then located. The WTRU then selects a new AP from among the APs from which it receives beacon packets. Both active and passive scanning can take a significant amount of time in terms of being able to maintain an ongoing communication, particularly when the WTRU is moving.
The inventors have recognized that it is sometimes desirable that an AP take the initiative of reassociating a WTRU. For example, APs (or some other node controlling the WTRU's behavior) may determine a current “load” condition based on the collective volume of wireless communication traffic due to the number of WTRUs associated with the AP an/or the types of communications being conducted. Also a WTRU may be located at location served by several neighboring APs each of which may be using a different channel from among the set of channels over which the WTRU can communicate. Where the collective traffic volume is relatively high (heavy load) for one AP and the volume of traffic on one or more neighboring APs is relatively low (light load), it would be desirable that one or more WTRUs associated with the heavily loaded AP re-associate with a neighboring lightly loaded AP.
In WLANs compliant with the current 802.11 standards, however, these re-associations take place only if WTRUs associated with the heavily loaded AP are equipped with a sophisticated algorithm allowing them to autonomously decide re-association to another available AP is desirable, based on traffic estimations on the channels. It is difficult to be certain that all WTRUs would be equipped with such an algorithm, and the design and implementation of such an algorithm can be quite complex. Even if all WTRUs operating in a given geographical area are equipped with such an algorithm, it is difficult to guarantee that the WTRUs would not re-associate to other APs in a disorderly fashion, possibly resulting in multiple re-associations taking place back and forth between the same APs.
To avoid these issues, the inventors have considered potential solutions. For example, a heavily loaded AP could forcefully disassociate some selected WTRUs, hoping that these WTRUs would then find another AP with which they can re-associate. It could also induce disassociations indirectly by reducing the transmission power level of certain key packets it transmits, for example, beacon or probe response packets. The problem with these disassociation techniques is that they can result in a disruption of service unacceptable to certain WTRU users, for example, users of real-time services such as voice or video. This disruption is caused by the WTRU having to scan for a new AP after disassociation, a process which can take a significant amount of time. In consideration of the overall problems, the inventors have recognized it would be desirable to provide for allowing an AP to initiate the re-association of a WTRU from one AP to another without loss of performance.